In an electrophotographic image forming apparatus, a corona charging method has been conventionally employed as a contact-free charging method. According to the corona charging method, ions are generated by corona discharging, and led to a surface of an image bearing member such as a photoreceptor drum 3 so as to electrically charge it.
Meanwhile, a corona charging apparatus gives rise to a problem that nitrate ions generated by the corona discharge migrate to and adhere to the photoreceptor drum 3 while an image forming operation is stopped, thereby causing an image failure such as white spot to an outputted image.
This problem is described in more detail with reference to FIG. 13. FIG. 13 is an explanation view schematically showing a white-spotted image generation mechanism in the photoreceptor drum 3.
As shown in FIG. 13, ozone (O3) is generated by the corona discharge in the charging apparatus 5. Resultant Ozone (O3) oxidizes nitrogen in the air so as to produce nitrogen oxide (NOX). This nitrogen oxide (NOX) is reacted with moisture in the air so as to be changed to nitric acid (HNO3). Nitric acid produced as such adheres to the surface of the photoreceptor drum 3 near the charging apparatus 5 while the image forming apparatus is left after the image forming operation. Nitrate ions change a surface resistance of an adhered part of photoreceptor drum 3. As a result, a charge retention capability of the photoreceptor drum 3 is decreased at this part. When the image formation is later carried, a white-spotted image is caused due to the decreased charge retention capability.
In order to solve such a problem, a conventional corona discharging apparatus includes a fan for discharging an air from an inside of the corona discharging apparatus. This configuration decreases an amount of NOX remaining in the corona discharging apparatus after the image forming operation. The configuration thereby decreases an amount of nitric acid that adheres to the photoreceptor drum 3 when the image forming apparatus is left after the image forming operation.
Patent literature 1 discloses a technique in which aluminum hydroxide is applied to an inner surface of a shield case which inner surface faces a discharging electrode, so as to form an aluminum hydroxide coating having no hole and substantially unbroken. The aluminum hydroxide of the coating causes an irreversible neutralization of NOX.